1. Field of the Invention
The invention is related to the field of satellite tracking, and in particular to apparatus and methods of using multiple laser beams to generate optical beacons for low earth orbital satellite tracking of ground stations in optical communication with the satellite.
2. Description of the Prior Art
In order for a satellite to optically communicate with a ground station, it must be able to orient its antenna, which is often highly directional, toward the ground station. The ground station must therefore send up a directional beacon to the satellite on which the satellite can lock for orientation purposes.
Free space optical communications requires a beacon laser for acquisition and tracking of the ground station by the spacecraft terminal. However, a laser beam propagated through the atmosphere experiences scintillation and beam wander which breaks up the beam and causes signal fades at the receive system. This intermittent loss of signal breaks the tracking lock and requires reacquisition of the beacon laser source.
Uniform illumination of a distant object, such as a missile target, or a satellite ground station by a remote laser is prevented by the scintillation of the laser beam as it traverses the turbulent atmosphere to reach the target. The laser beam is essentially passed through a large number of randomly-oriented, time-varying prisms in the atmosphere which break the beam into many beamlets with slightly different directions. As these coherent (all having originated from a coherent wavefront of diameter D at the beam directing telescope) beamlets arrive at the target with random time-varying position, they interfere to provide a large variation of intensity with position on the target.
It has been speculated for some time that the use of many (say n) small mutually incoherent laser beams with diameter ≦r0, where r0 is the so-called “atmospheric coherence diameter” as defined by astrophysicist David Fried, separated spatially but originating within the same area A=.ΠD2/4, where D was the diameter of the single, coherent beam at the projection telescope, would, if focused to the target, provide more uniform illumination than that of the single beam. For a complete discussion of the atmospheric coherence diameter, r0, and other features of atmospheric turbulence and compensation, see Atmospheric-Compensation Technology, J.Opt.Soc.Am., (R. Benedict, Jr., J. Breckinridge, David Fried, Editors) A, Vol. 11, No. 1, January 1994.
Lucent, Astroterra, Terrabeam and MIT Lincoln Labs among others, are involved in free space optical communication for terrestrial applications. Astrorerra makes use of a four beam beacon laser assembly. Lincoln Lab has used multi-beam transmission from a single laser during active missile tracking disclosed in U.S. Pat. No. 5,734,504 discussed below. Thus, multi-beam laser tracking is known for satellite applications.
A multi-beam illuminator laser made by Lockheed Martin Corp. is shown in U.S. Pat. No. 5,734,504. The multi-beam illuminator laser is intended to provide a uniform laser beam illumination of a distant target or remote object, even in the presence of changing atmospheric conditions. A diameter-adjustable array provides a variable number of co-parallel, mutually incoherent, polarization-aligned, waist size- and position-adjustable beamlets. A beam divider uses input or source laser power and because of this may be driven by as few as one laser or by as many as n lasers, where n is equal to the beamlet number. Rapid adjustment of the beamlet number and beamlet positions in the field allows determination of the optimum number of beamlets to use. Finally, as the total number of beamlets and possibly their positions are varied, the individual beamlet powers are maintained equal to each other and the overall beamlet array power is easily held constant by choice of the stage angles within the beam divider.